Cylinder head structure of engine

ABSTRACT

The first flow-restriction vertical bead narrowing the coolant flow is provided to increase the flow speed at this portion. The coolant passage opens at the flow-in area between the exhaust ports. Thereby, the flow speed of the coolant at this area is increased, so the coolant in the coolant passage can be properly sucked due to the Venturi effect. As a result, the cooling function can be improved.

BACKGROUND OF THE INVENTION

The present invention relates to a cylinder head structure of an engine, particularly to a cylinder head of an engine that can improve the engine cooling function.

An engine valve drive mechanism comprising two intake valves and two exhaust valves provided for each cylinder has been recently adopted to improve an intake and exhaust efficiency of the engine.

The engine equipped with such a valve drive mechanism has been also desired to have a higher power as well as compactness of its engine body. Accordingly, some measures against a severe heat load due to the higher power have been required.

In particular, it is important to improve the cooling function around the combustion chamber and the exhaust ports, and therefore some technologies for those have been proposed.

For example, Japanese Patent Laid-Open Publication 2003-314357 discloses a structure that the cross section area of the plug tower portion (the cylindrical ignition-plug hole wall) of the cylinder head is configured to be gradually enlarged from the combustion-chamber side to the upper-deck side (from below to above) to increase the coolant flow speed at the combustion-chamber side in the water jacket compared with that at the upper-deck side, thereby improving the cooling function of the engine.

Meanwhile, since the total flow volume of the engine coolant is determined based on the function of the water pomp etc., the efficiency of the cooling function rests upon how to make the coolant flow.

Also, the engine generally has different cooling requirements, namely the exhaust-side portion requires more cooling than the intake-side portion does. The art disclosed in the above-described publication, however, just attempts to increase the coolant flow speed at the combustion-chamber side entirely in the water jacket, and so would not provide more appropriate cooling function for respective portions.

Further, it is necessary to reduce properly the temperature around the ignition plug, but the art in the publication would not consider the cooling function of the ignition-plug hole wall itself either.

In addition, a drill hole is generally formed to make the coolant passage for introducing the coolant from the cylinder block into the portion between the exhaust ports so as to cool this portion properly. However, any measures for properly cooling the portion are not disclosed in the above-described art.

SUMMARY OF THE INVENTION

The present invention has been devised in view of the above-described problems, and an object of the present invention is to provide the cylinder head structure of an engine that can improve the cooling function of the exhaust-side portion efficiently as well as the intake-side portion, providing the proper cooling function of the ignition-plug hole wall.

According to the present invention, there is provided a cylinder head structure of an engine, in which the engine has multiple cylinders, there is provided for each cylinder a valve unit comprising two intake valves and two exhaust valves and an ignition plug located at a substantially central portion of the cylinder, and there are provided a main water jacket to allow part of a coolant to flow around each ignition-plug hole wall in a cylinder-row direction and an exhaust-side sub water jacket to allow part of the coolant to flow around each exhaust port wall in the cylinder-row direction, the both water jackets being connected to each other, the cylinder structure comprising a first vertical bead portion that is formed at a portion of the ignition-plug hole wall that is opposed to the exhaust port wall, the first vertical bead portion providing a smallest cross section of a passage between the ignition-plug hole wall and the exhaust port wall in said main water jacket, a second vertical bead portion that is formed at a portion of the ignition-plug hole wall that is opposed to an intake port wall, the second vertical bead portion providing a smallest cross section of a passage between the ignition-plug hole wall and the intake port wall in the main water jacket, and a coolant passage that is provided between a pair of exhaust port walls of the cylinder, one end of the coolant passage opening at a cylinder-head lower face and the other thereof opening at a location that is opposed to the ignition plug hole wall, wherein the smallest cross section provided by the first vertical bead portion is configured so as to be greater than the smallest cross section provided by the second vertical bead portion, and the first vertical bead portion formed at the ignition-plug hole wall is configured so as to be opposed to the exhaust port wall that is located at an upstream side of the flow direction of the coolant.

Accordingly, since the smallest cross section at the exhaust-side portion of the main water jacket is configured to be greater than that at the intake-side portion of the main water jacket by the first and second vertical bead portions, the coolant flow volume at the exhaust-side portion in the jacket can be made greater than that at the intake-side portion, suppressing the total coolant flow volume of the main water jacket properly.

Also, since the first and second vertical bead portions are exposed on the coolant flowing, the cooling function of the ignition-plug hole wall can be also improved.

Further, since the first vertical bead portion formed at the ignition-plug hole wall is configured so as to be opposed to the exhaust port wall that is located at the upstream side of the coolant passage, the flow of the coolant is narrowed upstream of the portion where the above-described the coolant passage opens. Thereby, the flow speed of the coolant at the portion where the coolant passage opens is increased, so the coolant in the coolant passage can be properly sucked in the portion due to the Venturi effect. As a result, more coolant is provided around the exhaust-side portion and the cooling function around there can be improved.

Herein, shapes of the first and second vertical bead portions should not be limited to a particular shape, but a smoother configuration may be appropriate to increase the flow speed of the coolant.

Also, as long as it can narrow the flow of the coolant at its location being opposed to the intake port wall, the second vertical bead portion may be provided so as to be opposed to any one of a pair of intake ports.

According to an embodiment of the present invention, an exhaust-side passage portion restricted by the first vertical bead portion is configured so as to be higher in a vertical direction than an intake-side passage portion restricted by the second vertical bead portion, and the first vertical bead portion is configured so as to be longer in the vertical direction than the second vertical bead portion.

Accordingly, the area where the coolant speed increases by the exhaust-side passage portion can be made wider over the vertical direction from a lower position to an upper position.

Thereby, the suction of the coolant via the portion from the cylinder block side to the portion between a pair of exhaust portions (from below to above obliquely) can be promoted by enlarging the area where the Venturi effect prevails.

As a result, the cooling function by the coolant passage between the exhaust ports can be increased and thereby the cooling function can be improved effectively.

According to another embodiment, the exhaust-side passage portion restricted by the first vertical bead portion has a substantially constant width over a height thereof.

Accordingly, the increase of the flow speed of the coolant by the first vertical bead portion can be provided uniformly over the whole area at the exhaust-side portion.

Thus, the coolant is properly moved even at the combustion side (lower side), so the cooling function at the combustion side can be improved.

As a result, the cooling function at the combustion side that may receive severe heat load can be improved.

According to another embodiment of the present invention, the second vertical bead portion formed at the ignition-plug hole wall is configured so as to be opposed to the intake port wall that is located at a downstream side of the flow direction of the coolant.

Thus, since the second vertical bead portion is formed at the portion that is opposed to the intake port wall located at the downstream side, the coolant flowing in the area between a pair of intake ports would not be affected improperly by the second vertical bead portion.

Thus, since the coolant streams down smoothly through the area between the intake ports without stagnating, the proper cooling function at the intake-side portion can be obtained.

Namely, in the case where no coolant passage is formed at the area between the intake ports unlike the area between the exhaust ports described above, the stagnation of the coolant would be promoted in this area. According to the above-described embodiment, however, second vertical bead portion is located at the downstream side, so the occurrence of the stagnation could be prevented appropriately.

According to another embodiment of the present invention, there is provided an intake-side sub water jacket that is located below the intake ports of each cylinder to connect with the main water jacket, a fuel-injector hole wall to accommodate a fuel injector is formed at a portion that is between a pair of intake ports and below the intake ports, and the fuel-injector hole wall is located in the intake-side sub water jacket.

Accordingly, since the intake-side sub water jacket is provided below the intake ports and the fuel-injector hole wall is located in the intake-side sub water jacket, the fuel injector can be cooled properly with the coolant in the intake-side sub water jacket. Thus, the heat load to the fuel injector can be reduced.

Particularly, since the flow volume in the main water jacket is narrowed by the first and second vertical bead portions and the coolant volume flowing into the intake-side sub water jacket is increased instead, the cooling of the fuel injector can be improved surely.

As a result, the cooling of the inside of the cylinder head with a limited volume of coolant can be attained efficiently.

Other features, aspects, and advantages of the present invention will become apparent from the following description which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a coolant path of an engine with a cylinder head according to the present invention.

FIG. 2 is a plan view of the partially-cut-out cylinder head.

FIG. 3 is a sectional view taken along line A-A of FIG. 2.

FIG. 4 is a sectional view taken along line B-B of FIG. 2.

FIG. 5 is a sectional view taken along line C-C of FIG. 2.

FIG. 6 is a bottom face view of the cylinder head.

FIG. 7A is a plan view of the cylinder block, and FIG. 7B is a plan view of a gasket.

FIG. 8 is a sectional view taken along line D-D of FIG. 2.

FIG. 9 is a plan view of part of a main water jacket located around an ignition-plug hole wall.

FIG. 10 is perspective view showing around the ignition-plug hole wall, when viewed from the exhaust side.

FIG. 11 is perspective view showing around the ignition-plug hole wall, when viewed from the intake side.

FIG. 12 is a sectional view taken along line F-F of FIG. 3.

DETAINED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the present invention will be described referring to the accompanied drawings. FIG. 1 is a schematic diagram of a coolant path X of an engine E with a cylinder head according to the present invention.

The coolant path X of the engine E comprises a coolant circulation path that includes a radiator 1 to cool heated coolant, a thermostat 2 to control the flow volume of the coolant, a water pump 3 to supply the coolant in the path, a cylinder block 4 to be cooled by the coolant, and a cylinder head 5 to be cooled by the coolant likewise. The engine E is cooled by the coolant that is circulated in this path. Herein, a cooling fan 6 is provided to cool the radiator 1.

The flow volume of the coolant in the coolant path X is adjusted to a specified volume by the water pump 3 and others. Thus, the cooling function of the engine E depends on how to control distribution of the coolant in the path.

According to the present embodiment, a proper structure is provided to the cylinder head 5 of the engine E, thereby increasing the efficient flow of the coolant and improving the cooling function of the engine E.

FIG. 2 is a plan view of the partially cut-out cylinder head 5. FIG. 3 is a sectional view taken along line A-A of FIG. 2 (sectional view between ports). FIG. 4 is a sectional view taken along line B-B of FIG. 2 (sectional view between ports). FIG. 5 is a sectional view taken along line C-C of FIG. 2 (sectional view between cylinders). FIG. 6 is a bottom face view of the cylinder head 5. FIG. 7A is a plan view of the cylinder block 4, and FIG. 7B is a plan view of a gasket 7.

The cylinder head 5 of the present embodiment is the one of the 4-cylinder inline engine E. The cylinder head 5 is assembled on the cylinder block 4 via the gasket 7.

Hereinafter, a longitudinal direction, namely a cylinder-row direction of the cylinder heard 5 is referred to as the engine longitudinal direction, an output end side of a crank shaft (upper side in FIG. 2) is referred to as the engine rear side, its opposite side (lower side in FIG. 2) is referred to as the engine front side, a left side when viewed from the back (right side in FIG. 2) is referred to as the engine intake side, and its opposite side (left side in FIG. 2) is referred to as the engine exhaust side. Likewise, respective sides of the engine E are defined by these in FIGS. 6 and 7.

At a bottom deck 5 a of the cylinder head 5 is formed a combustion-chamber ceiling portion 11 that closes a cylinder 10 (see FIG. 7) of the cylinder block 4 from above, as shown in FIGS. 3 and 4.

The combustion-chamber ceiling portion 11, which is formed in a so-called pent-roof shape as shown in FIG. 3, has at the center thereof a pug hole 12 to accommodate an ignition plug (not illustrated) to be inserted vertically along an axis of the cylinder.

Also, there are provided intake ports 13 at the intake side and exhaust ports 14 at the exhaust side respectively so as to open at slat portions of the combustion-chamber ceiling portion 11 of each cylinder enclosing the center plug hole 12, as shown in FIG. 2. Intake and exhaust valves 15 a, 15 b (shown in two-dotted broken lines in FIG. 4) are respectively disposed at their ports 13, 14.

The intake ports 13, 13 are provided so as to extend substantially straight and upward obliquely from the combustion chamber and to open at the intake-side portion of the cylinder head 5 independently as shown in FIG. 4. Meanwhile, the exhaust ports 14, 14 are provided so as to merge with each other at a specified point and extend substantially horizontally and to open at the exhaust-side portion of the cylinder head 5.

At the intake side of the bottom deck 5 a is formed a nozzle hole 16 for accommodating a fuel injector (not illustrated) to inject fuel into the combustion chamber as shown in FIG. 3.

As shown in FIG. 6, respective cylinders 10 are located so close to each other that respective portions 17 . . . of the bottom deck 5 a that are located between the cylinders are thin (narrow) in the cylinder-row direction.

The cylinder head 5, as shown in FIG. 3, has a middle deck 5 b at a substantially middle position thereof in the vertical direction. Above the middle deck 5 b are provided intake and exhaust camshafts (not illustrated), while below the middle deck 5 b are provided a head-side water jacket 20 that is partitioned by the bottom deck 5 a, jacket side walls 18 and the like.

Also, at the middle deck 5 b are provided valve holes 19 a, 19 b for the intake and exhaust valves 15 a, 15 b of each cylinder at both sides of the plug hole 12. Also, head-bolt through holes 22 and bolt boss portions 21 for head bolts (not illustrated) to assemble the cylinder head 5 on the cylinder block 4 are formed around the cylinder.

The intake and exhaust camshafts (not illustrated) to drive respective intake and exhaust valves 15 a, 15 b are disposed above the middle deck Sb so as to extend in parallel in the engine longitudinal direction at respective portions just above the valve holes 19 a, 19 b.

The cylinder head 5 further has journal potions 23, 24 that are located at both sides of the plug hole 12 of each cylinder, which are supports of the respective camshafts.

The above-described head-side water jacket 20, which is provided at the central portion in the engine width direction above the bottom deck 5 a and the intake and exhaust ports 13, 14 as shown FIG. 4 and others, includes a main water jacket 31 that extends in the cylinder-row direction from a foremost first combustion-chamber ceiling portion 11 to a rearmost fourth combustion-chamber ceiling portion 11, an intake-side sub water jacket 32 that extends in the cylinder-row direction between the intake ports 13 . . . and the bottom deck 5 a, and an exhaust-side sub water jacket 33 that extends in the cylinder-row direction between the exhaust ports 14 . . . and the bottom deck 5 a. The water jackets 31 and 32 are connected with each other via branch passages 34 . . . etc. Likewise, the water jackets 31 and 33 are connected with each other.

The main water jacket 31 is partitioned by the middle deck 5 b at its upper portion and by the bottom deck 5 a at its lower portion as shown in FIG. 4. Further, the sides of the main water jacket 31 are partitioned by an intake port wall 25 forming the intake ports 13 . . . , an exhaust port wall 26 forming the exhaust ports 14 . . . , respective bolt bosses 21 and the like. At the central portion of the main water jacket 31 in the engine width direction is provided a plug hole wall 27 forming the plug hole 12 that stands vertically.

Accordingly, the coolant flows down in the main water jacket 31 from the engine front to the rear in the cylinder-row direction, along the respective intake port walls 25, exhaust port walls 26, and plug hole walls 27.

The above-described intake-side sub water jacket 32 is formed, as shown in FIG. 3, between the intake port wall 25 and a nozzle hole wall 28 forming the nozzle hole 16 of the injector, and extends in the cylinder-row direction between the front and rear of the engine E. Like the main water jacket 31, the coolant flows down in this water jacket 32 from the engine front to the rear in the cylinder-row direction along the respective intake port walls 25 and the nozzle hole walls 28.

The above-described exhaust-side sub water jacket 33 is formed, as shown in FIG. 4, between the intake port wall 26 and the bottom deck 5 a, and extends in the cylinder-row direction between the engine front and the rear. Like the other water jackets, the coolant flows down in this water jacket 33 from the engine front to the rear in the cylinder-row direction along the respective exhaust port walls 26.

Also, as shown in FIG. 3, there is provided between a pair of exhaust ports 14, 14 a coolant passage 35 made of a drill hole to interconnect a water jacket of the cylinder block 4 and the head-side water jacket 20 for introducing the coolant from the cylinder block 4 into the portion between the exhaust ports 14, 14.

Thus, by the coolant passage 35 actively introducing the coolant from the cylinder block 4 into the portion between the exhaust ports 14, 14, the cooling function of the portion between the exhaust port walls 26 that are exposed to burned gas with a high temperature can be improved.

The four cylinders 10 . . . are formed at the cylinder block 4 in the cylinder-row direction as shown in FIG. 7A, and water jackets, not illustrated, are provided around the respective cylinders for cooling. Also, a plurality of peripheral openings 41 . . . are formed at peripheral portions around the cylinders at the top deck. At both sides of the front end of the top deck are provided intake-side and exhaust-side coolant-introduction openings 42, 43, respectively.

Herein, there are provided pins 45 between each cylinder at the engine intake exhaust sides, which function as positioning of the cylinder head 5 along with bolt holes 44 fastened.

Meanwhile, as shown in FIG. 7B, the gasket 7 has four cylinder holes 71 corresponding to the respective cylinders, and through holes 72 for the head bolts passing through. Also, it has connection holes 73, 74 corresponding to the above-described coolant-introduction openings 42, 43 at the engine front side.

These connection holes 73, 74 are formed such that the exhaust-side connection hole 74 has a larger hole area than the intake-side connection hole 73 does, which is apparent from FIG. 7B. Thereby, the coolant flow volume at the exhaust side is increased.

Also, the gasket 7 includes connection holes 75 . . . are formed corresponding to the peripheral openings 41. The opening area of the connection hole 75 is configured to be considerably smaller than that of the connection holes 73, 74, so most of the coolant flowing into the cylinder head 5 from the cylinder block 4 passes through the connection holes 73, 74.

Further, the opening area of the exhaust-side connection hole 75 is configured to be greater than that of the intake-side connection hole 75. This is because the coolant needs to be introduced via the above-described coolant passage 35 at the exhaust side.

Thereby, the flow position and flow volume of the coolant flowing into the cylinder head 5 from the cylinder block 4 is determined.

Meanwhile, the water jacket at the side of the cylinder head 5 receiving the coolant from the openings 73, 74 and 75 of the gasket 7 includes intake-side and exhaust-side openings 51, 52 formed at the engine front, and a plurality of peripheral openings 53 . . . formed at peripheral portions of the cylinders, as shown in FIG. 6.

Herein, the openings 51, 52 receive the coolant from the cylinder block 4 mainly, and the coolant received via these openings flows into the main water jacket 31, and then the exhaust-side sub water jacket 33 and the intake-side sub water jacket 32. Although some coolant flows in from the peripheral openings 53 . . . through the connection openings 75 . . . of the gasket 7, the amount of this flow is very little. Thereby, the coolant flowing in the cylinder head 5 happens surely from the engine front to the rear.

According to the cylinder head 5 of the present embodiment, the flow volume and speed of the coolant in the main water jacket 31 is adjusted properly. This will be described referring to FIGS. 8-11. FIG. 8 is a sectional view taken along line D-D of FIG. 2, showing the cross section of the plug hole wall 27. FIG. 9 is a plan view of part of the main water jacket 31 located around the plug hole wall 27. FIG. 10 is perspective view showing around the plug hole wall 27, when viewed from the exhaust side. FIG. 11 is perspective view showing around the plug hole wall 27, when viewed from the intake side. In these figures, each arrow shows a flow direction of the coolant.

As shown in FIG. 9 and others, a first flow-restriction vertical bead 81 is formed at a portion 27 a of the plug hole wall 27 that is opposed to the exhaust port wall 26 located at the engine front side (the coolant-flow upstream side), and a second flow-restriction vertical bead 82 is formed at a portion 27 b of the plug hole wall 27 that is opposed to the intake port wall 25 located at the engine rear side (the coolant-flow downstream side) (see FIG. 11).

The first flow-restriction vertical bead 81 is formed so as to extend relatively long vertically from a lower position to a higher position at the plug hole wall 27 as shown in FIG. 8, and its cross section has an arc shape with a gentle curvature so that a coolant passage 31 a is narrowed gradually between this bead 81 and the circular-shaped exhaust port wall 26.

Meanwhile, the second flow-restriction vertical bead 82 is formed so as to extend relatively short vertically compared to the first flow-restriction vertical bead 81 as shown in FIG. 8, and likewise its cross section has the arc shape with the gentle curvature so that a coolant passage 31 b is narrowed gradually between this bead 82 and the circular-shaped intake port wall 26.

Thus, since the total follow volume of the coolant in the main water-jacket 31 is restricted (narrowed) by these first and second flow-restriction vertical beads 81, 82, the volume of the coolant flowing out from the branch passages 34 ino the intake-side and exhaust-side sub water jackets 32, 33 can be increased. Also, since these beads 81, 82 are formed at the plug hole walls 27, the cooling function of the plug hole walls 27 can be improved.

Further, as apparent from FIG. 8, a smallest passage cross area S1 of the exhaust-side coolant passage 31 a is set to be greater than a smallest passage cross area S2 of the intake-side coolant passage 31 b.

Thus, the coolant flow volume of the exhaust-side coolant passage 31 a can be made greater than that of the intake-side coolant passage 31 b, and thereby the coolant flow volume in the exhaust-side sub water jacket 33 can be made greater than that in the intake-side sub water jacket 32, suppressing the total coolant flow volume in the main water jacket 31. Accordingly, the cooling function at the exhaust side can be improved.

Particularly, the cooling function can be further improved by providing the first flow-restriction vertical, bead 81 at this portion for the following reason.

Namely, as shown in FIG. 9, since the first flow-restriction vertical bead 81 is provided at this location, the flow of the coolant is narrowed and thereby the flow speed of the coolant at this portion. Since the coolant passage 35 opens at a flow-in area 31 c between the exhaust ports 14, 14 as described above, the coolant in the coolant passage 35 can be properly sucked in this area due to the Venturi effect. Thereby, the cooling function at the exhaust side can be improved.

Also, since the first flow-restriction vertical bead 81 is formed so as to extend widely (long) in the vertical direction as described above, the Venturi effect can be created over the wide (long) range and thereby the coolant sanction function of the coolant passage 35 from the cylinder block 4 can be further improved.

Further, since the exhaust-side coolant passage 31 a restricted by the first flow-restriction vertical bead 81 formed vertically long is configured so as to have a substantially constant width w1 over its height, the coolant flow can be created at the combustion-chamber side (lower side) as well. Accordingly, the proper coolant flow can be surely generated and thereby the cooling function at the combustion-chamber side can be improved.

Meanwhile, the second flow-restriction vertical bead 82 is formed at the portion 27 b of the plug hole wall 27 that is opposed to the intake port wall 25 located at the engine rear side (the coolant-flow downstream side) as described above. Accordingly, the cooling function at the intake side can be improved for the following reason.

In the case where the second flow-restriction vertical bead 82 is formed at a portion of the plug hole wall 27 that is opposed to the intake port wall 25 located at the engine front side, the coolant flow speed would increase partially at a flow-in area 31 d between the intake ports 13, 13. And, in the case where no coolant passage 35 is provided between the intake ports 13, 13, unlike between the exhaust ports 14, 14, the cooing would not be done properly at this area.

Namely, this partial flow speed increasing may generate “vortex” in the flow-in area 31 d as shown by a broken line in FIG. 9, so that stagnation of the coolant would occur in this area.

According to the above-described embodiment, however, the second flow-restriction vertical bead 82 is formed at the portion that is opposed to the intake port wall 25 located at the engine rear side, thereby not changing the flow speed of the coolant in this flow-in area 31 d. Thus, the proper and uniform coolant flow can be obtained and thereby the proper cooling function can be provided.

Further, as shown in FIG. 8, the intake-side coolant passage 31 b restricted by the second flow-restriction vertical bead 82 is configured so as to have a substantially constant width w2 over its height. Thereby, the proper coolant flow can be surely generated and the cooling function at the combustion-chamber side can be improved.

Further, since the coolant flow volume in the main water jacket 31 is narrowed by the first and second flow-restriction vertical beads 81, 82 and thereby the coolant volume flowing into the intake-side sub water jacket 32 is increased, the cooling of the nozzle hole wall 28 located in the intake-side sub water jacket 32 can be improved surely.

Namely, as shown in FIG. 12, a sectional view taken along line F-F of FIG. 3, the intake-side sub water jacket 32 is formed so as to surround the nozzle hole wall 28 for the funnel injector. The fuel injector can be cooled properly by increasing the coolant flow volume to the intake-side sub water jacket 32.

Thus, according to the present embodiment, since the first and second flow-restriction vertical beads 81, 82 are provided respectively at appropriate portions of the plug hole walls 27, the cooling function of the cylinder head 5 can be improved properly and efficiently with the restricted volume of the coolant.

Particularly, since the first flow-restriction vertical bead 81 is formed at the portion opposed to the exhaust port wall 26 located at the engine front side (the coolant flow upstream side), the proper suction of the coolant from the coolant passage 35 can be obtained, thereby improving the cooling function at the exhaust side.

The present invention should not be limited to the above-described embodiment. For example, the second flow-restriction vertical bead 82 may be formed at a portion of the plug hole wall 27 that is opposed to the intake port wall 25 located at the engine front side (the coolant flow upstream side). Although there may occur some stagnation of the coolant in the flow-in area 31 d in this case, a sufficient flow volume of the coolant may reduce this stagnation. As a result, the cooling function by such a location of the second flow-restriction vertical bead 82 would not deteriorate improperly with the sufficient flow volume of the coolant.

Also, the shape or length of these beads 81, 82 may be modified properly according to factors such as the flow volume of the coolant.

In the correspondence between the structure of the present invention and the above-described embodiment, the first vertical bead portion corresponds to the first flow-restriction vertical bead 81, the second vertical bead portion corresponds to the second flow-restriction vertical bead 82, the ignition-plug hole wall corresponds to the plug hole wall 27, the exhaust-side passage portion corresponds to the exhaust-side coolant passage 31 a, the intake-side passage portion corresponds to the intake-side coolant passage 31 b, the fuel-injector hole wall corresponds to the nozzle hole wall 28.

The present invention should be applied to a cylinder head structure of any types of engines. 

1. A cylinder head structure of an engine, in which the engine has a multiple cylinders, there is provided for each cylinder a valve unit comprising two intake valves and two exhaust valves and an ignition plug located at a substantially central portion of the cylinder, and there are provided a main water jacket to allow part of a coolant to flow around each ignition-plug hole wall in a cylinder-row direction and an exhaust-side sub water jacket to allow part of the coolant to flow around each exhaust port wall in the cylinder-row direction, the both water jackets being connected to each other, the cylinder structure comprising: a first vertical bead portion that is formed at a portion of the ignition-plug hole wall that is opposed to the exhaust port wall, the first vertical bead portion providing a smallest cross section of a passage between the ignition-plug hole wall and the exhaust port wall in said main water jacket; a second vertical bead portion that is formed at a portion of the ignition-plug hole wall that is opposed to an intake port wall, the second vertical bead portion providing a smallest cross section of a passage between the ignition-plug hole wall and the intake port wall in said main water jacket; and a coolant passage that is provided between a pair of exhaust port walls of the cylinder, one end of the coolant passage opening at a cylinder-head lower face and the other thereof opening at a location that is opposed to the ignition plug hole wall, wherein said smallest cross section provided by the first vertical bead portion is configured so as to be greater than said smallest cross section provided by the second vertical bead portion, and said first vertical bead portion formed at the ignition-plug hole wall is configured so as to be opposed to the exhaust port wall that is located at an upstream side of the flow direction of the coolant.
 2. The cylinder head structure of an engine of claim 1, wherein an exhaust-side passage portion restricted by said first vertical bead portion is configured so as to be higher in a vertical direction than an intake-side passage portion restricted by said second vertical bead portion, and said first vertical bead portion is configured so as to be longer in the vertical direction than said second vertical bead portion.
 3. The cylinder head structure of an engine of claim 2, wherein said exhaust-side passage portion restricted by the first vertical bead portion has a substantially constant width over a height thereof.
 4. The cylinder head structure of an engine of claim 1, wherein said second vertical bead portion formed at the ignition-plug hole wall is configured so as to be opposed to the intake port wall that is located at a downstream side of the flow direction of the coolant.
 5. The cylinder head structure of an engine of claim 1, wherein there is provided an intake-side sub water jacket that is located below the intake ports of each cylinder to connect with said main water jacket, a fuel-injector hole wall to accommodate a fuel injector is formed at a portion that is between a pair of intake ports and below the intake ports, and said fuel-injector hole wall is located in said intake-side sub water jacket. 